Which type motor to use and driver?

Thanks to all who might give me some input on this decision.

I want to make my large garden tractor, small tractor steer in a straight line. (this will be used in planting trees, watering, spraying, etc, not mowing.) I have a compass setup and working dead steady on the machine.

It doesn't take much to turn while moving, I can do it with 1 finger. I'm leaning towards a stepper motor as I am familiar with them. I'm wondering about the torque of DC motor compared to a stepper and the difference in controlling.

So the question is, should I use a stepper or a DC motor? Why?

Along with your motor suggestion, which driver would you suggest?


Based on what you have told us I don't understand the fine detail of the control system you have in mind.

For example if you detect an error in the heading and know the exact degrees of steering correction to apply then a stepper motor would be useful.

On the other hand if you propose to add some (small) steering correction and then review the effect on the heading before deciding if a greater correction is needed then I reckon a DC motor would probably do just as well.

In either case you should measure the required torque in the most difficult condition and then get a motor wilt at least a 50% excess capability. There is a simple method for measuring torque in Stepper Motor Basics

Only when you have identified the motor will you be in a position to identify a driver - whichever type of motor you opt for.

Separately, I would be surprised if a compass is sufficient to give you the straight-line precision you require. I know that agricultural tractors now use some sort of GPS based steering system but my guess is that it is augmented by other sensors and some sophisticated software. For your situation some sort of visible beacon at then end of a row may be worth considering. Of course I may be all wrong about this.


is your compass from the autopilot on a boat? With switches to tell when to move the rudder left or right?

Have you considered just putting on the wheel brake to correct the direction?


The big boy tractor are using GPS or RTK. There is also a compass to provide course when standing still and until speed and travel distance is enough to get a fix from the GPS - RTK. RTK is extremely accurate and extremely expensive. GPS is inexpensive but is not very precise. The expert at the John Deere store locally told me the best bet is a compass.

I've only done limited testing at this time but It appears that I can be within a half inch on a 400' row. If that proves true it's good enough for what I'm doing.

Read the info on how to get the torque. That was helpful and where I will start.


It doesn't take much to turn while moving, I can do it with 1 finger.

Unfortunately that imparts little information to me - what you need to find out is the worst case torque or force needed
by the steering mechanism in physical units. I can pull 100N force or more with one finger, for instance,
you might be meaning 1 newton or 50 newtons, and at a radius of 5cm or 50cm, so I have not the slightest
idea of the force or torque involved, and that's the starting point for all calculations.

The torque of a DC gearmotor is determined by the gear ratio principally - you won't be need much
angular velocity so a big reduction factor is available. The important equation is:
power = torque x angular velocity (units W, Nm and rad/s respectively). Treat a gearbox as dropping
the power somewhat (friction losses), and thus trading speed for torque

For steppers the graph of torque v. speed will be in the datasheet, usually given for a range of driver supply
voltages. stepper motors with gearboxes are also available.

The expert at the John Deere store locally told me the best bet is a compass.

That guy may in fact know what he is talking about, but if so he is one of very very few knowledgeable showroom staff (IMHO).

Staff in PC stores who you would expect to know just a teensy bit about computers seem to know little apart from the colour.